The invention relates to a method and apparatus for leak-testing an electroluminescent device having at least one organic light-emitting diode, the device being enclosed in a housing. The invention also relates to an EL device suitable switch is for testing, or has been tested.
More particularly, the invention relates to testing an electroluminescent device comprising:
an electroluminescent element which comprises
an electroluminescent organic layer disposed between a hole-injecting electrode and an electron-injecting electrode; and
a housing which comprises
a first shaped part, a second shaped part and
an electrical leadthrough which contacts an electrode of said electroluminescent element;
said housing enclosing said electroluminescent element,
said electroluminescent element being mounted on said first shaped part,
said first and second shaped parts being connected to each other by means of a closed ring of a sealing material, and
a clearance being present between said electroluminescent element and said second shaped part.
An electroluminescent (EL) device is a device which, while making use of the phenomenon of electroluminescence, emits light when the device is suitably connected to a power supply. If the light emission originates in an organic material, said device is referred to as an organic electroluminescent device. An organic EL device can be used, inter alia, as a thin light source having a large luminous surface area, such as a backlight for a liquid crystal display or a watch. An organic EL device can also be used as a display if the EL device comprises a number of EL elements, which may or may not be independently addressable.
The use of organic layers as an EL layer in an EL element is known. Known organic layers generally comprise a conjugated, luminescent compound. Said compound may be a low-molecular type, forming an organic LED (O-LED), e.g. a low-molecular dye, such as a coumarin, or a high-molecular type type, forming a polymer LED (P-LED), such as a poly (phenylenevinylene). The EL element also comprises two electrodes, which are in contact with the organic layer. By applying a suitable voltage, the negative electrode, i.e. the cathode, will inject electrons and the positive electrode, i.e. the anode, will inject holes. If the EL element is in the form of a stack of layers, at least one of the electrodes should be transparent to the light to be emitted. A known transparent electrode material for the anode is, for example, indium tin oxide (ITO). Known cathode materials are, inter alia, Al, Yb, Mg:Ag, Li:Al or Ca. Known anode materials are, in addition to ITO, for example, gold and platinum. If necessary, the EL element may comprise additional organic layers, for example, of an oxadiazole or a tertiary amine, which serve to improve the charge transport or the charge injection.
An EL device of the O-LED type is disclosed in a publication by Burrows et. al., published in Appl. Phys. Lett. 65 (23), 1994, 2922. Said known device consists of an organic electroluminescent element which is built up of a stack of an ITO layer, an EL layer, a hole-transporting layer and a metal (in casu Mg:Ag) layer, which is provided with a silver layer. Said EL element is surrounded by a housing consisting of a bottom plate and a top plate which are made of glass, said plates being interconnected by an epoxy-based adhesive for sealing. Said ITO layer also forms the electrical leadthrough for the anode; the Mg:Ag/Ag layers also form the electrical leadthrough for the cathode. Said leadthroughs are insulated from each other by a layer of silicon nitride.
A polymer-LED device using a soluble poly (dialkoxy p-phenylenevinylene) as the emitting material is described in a publication by D. Braun et al., published in Synthetic Metals, 66 (1994) pp 75-79.
A percentage of the known devices has disadvantages which renders them unsuitable for use in durable consumer goods, such as a display or a backlight for a liquid crystal display or a watch. Already after several hours, the uniformity of the luminous surface deteriorates which can be observed with the unaided eye. Said deterioration, which also takes place when the EL device is not in operation, manifests itself, for example, by so-called xe2x80x9cdark spotsxe2x80x9d which are formed so as to be dispersed on the entire luminous surface.
Therefore, it is common practice to submit EL devices after manufacture to certain standard durability tests.
In a first test, i.e. a climate test, an EL device is immersed, under otherwise ambient conditions, in a water bath of 80xc2x0 C. for approximately 10 seconds and, immediately afterwards, in a melting ice bath for approximately 10 seconds. This procedure is repeated a number of times for 48 hours. After drying, a voltage of 6 V is applied to the electrodes as a result of which a luminous surface emitting light emerges. The luminous surface is checked on the presence of dark spots.
In a second test, a shelf life test, an EL device is stored at an elevated temperature and the luminance and the current are measured at regular intervals at a voltage of 6 Volt. For at least 650 hours, the constancy of the current and of the luminance are checked.
Lifetime tests of (organic) LEDs have shown that the exclusion of water from the devices is critical. To avoid contact with water, LED elements can be deposited on glass and covered with a metal hood (cover) sealed to the glass by means of an (organic) adhesive like an epoxy resin. An alternative is the use of a glass cover and a metal seal. The atmosphere inside the device can be Ar or N2. Because the sealing materials used up to now are not considered to be entirely impermeable to air, complete exclusion of water is difficult. This factor is likely to be the lifetime-determining factor with regard to shell life. To prolong the lifetime of a device, a powder may be added in the enclosure that serves as a getter for water. The getter may be, for example, a BaO getter or a CaO getter. The use of a getter, however, has a practical disadvantage for the manufacturer. It makes fast visual identification of devices with leaks difficult, as damage to the device in the form of black spots in the luminescent material is postponed. In practice the leaky devices will have a very short shelf life and they should be rejected before delivery to the customer.
It is therefore, one of the objects of the present invention to overcome, or reduce, the above disadvantage by providing a method which allows detection of leaks in electroluminescent devices without substantially any delay. The principal method of the invention for leak-testing an electroluminescent device having at least one light-emitting diode, the device being enclosed in a housing, involves local photo-oxidation of the LED material. In order that the test is not destructive, the area where the photo-oxidation takes place (by means of irradiation) should be outside the active (functional) electroluminescent area.
When air penetrates the encapsulating seal, water is immediately removed by a reaction with the (BaO) getter. As mentioned before, this makes the direct detection of water difficult. However, in the case of a leak, not only water but also oxygen will penetrate the seal and will form part of the atmosphere inside the device. The invention is based on the recognition that photo-degradation of luminescent (organic) materials is increased significantly in the presence of oxygen. This effect is used by irradiating a surface portion of the LED material with light of a predetermined wavelength and measuring whether the photo-luminescence signal of said surface portion of the LED material decreases. A decrease is an indication of a leak.
The photo-luminescence method is fast and can detect leaks in the seal in less than one minute, the duration in particular not exceeding 10 or even 3 seconds, which would take a day to be detectable by the occurrence of black spots. Oxygen concentrations as low as 40 ppm have been detected, so very small leaks can be detected.
Within the framework of the invention the above properties provide the possibility of an on-line leak-tester in the production process. In view of this, a second aspect of the invention provides an EL device suitable for testing as defined in claim 19, and a third aspect of the invention provides an EL device which has been tested as defined in claim 20.
In view of the above, a feature of the invention is to select a surface portion of the LED material which is in contact with the atmosphere in the housing, to irradiate said surface portion with light in a predetermined wavelength range, whereby the irradiated material emits light, to detect said emission, to generate an output signal which is proportional to an intensity of the detected emission, and to analyze said output signal on a decrease in time of an emission property. The irradiating light has a wavelength which brings the LED material to a luminescent state.
According to a further feature, the analysis takes place in a predetermined period of time. This period may advantageously be one minute or less, so that the leak testing can be done very quickly, allowing in particular on-line testing during manufacture.
The emission can be detected by using e.g. a spectrometer, but in a simple method of detecting the emission, a photo-electric detector can be used advantageously. The invention also relates to an apparatus for leak testing an electo-luminescent device having at least one light-emitting diode enclosed in a housing.